Electron spin resonance spectra are reported for the anions of tolan, stilbene, 1,2hyphen;bis(4hyphen;pyridyl)hyphen;ethylene, and azobenzene. In each experiment only one radical species was observed. The spectra were analyzed with the aid of thirdhyphen;derivative presentations and a nuclear magnetic resonance linehyphen;broadening experiment. All of the anions except tolan exhibit hindered rotation which allows the asymmetries of the spinhyphen;density distributions in the rings to be observed. Molecular orbital calculations with the McLachlan correction are in satisfactory agreement with the experimental hyperfine splitting constants. Methods for introducing asymmetry into the molecular orbital calculations are critically discussed and it is concluded that the sohyphen;called bgr; effect is probably the most desirable of the crude corrections. It is argued on the basis of Huuml;ckel molecular orbital theory that moderately rapid isomerization is reasonable for the stilbene anion.
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